Typically precious metal containing ores are leached with cyanide as the most efficient leachant or lixiviant for the recovery of precious metal values from the ore. It would also be highly desirable to recover nonprecious metal values by heap leaching or lixiviation.
However, because of the mineralogy of various ores, access to the precious and/or nonprecious metal in the ore by cyanide or other lixiviant is low for an economical extraction of the precious metal and/or nonprecious metal values in an ore. If the cyanide extraction produces small or negligible amounts of gold, an ore is said to be refractory or highly refractory. Various methods have been employed to increase the extractability of the precious and/or nonprecious metals. A good summary article describing the prior problems is that authored by Kantopoulos et al., Process Options for Refractory Sulfide Gold Ores: Technical, Environmental, and Economic Aspects, Proceedings EPO '90 Congress, D. R. Gaskell, Editor, The Minerals, Metals & Materials Society, 1990.
A typical component which causes the refractoriness of the ore is predominantly a carbonaceous type component either inorganic or organic. The organic carbonaceous materials are also classified as acid insoluble carbonaceous materials. Gold found in ores dispersed within or occluded in a sulfide matrix may be considered refractory because of inaccessibility of such gold by cyanide leaching. Similarly, nonprecious metal values found in ores either dispersed within or occluded in a sulfide matrix or present as metal sulfides are also not readily recoverable by heap leaching or lixiviation.
When treating such ores, the economic considerations dictate the selection of the process or the pretreatment of the ore to render it amenable first and foremost to cyanide extraction even though other gold lixiviants may be used. Similarly, it is highly desirable with nonprecious metal values in sulfidic ores to render them recoverable by heap leaching or lixiviation.
As one of the desired treatment steps prior to cyanidation or comparable lixiviation, roasting of ores in presence of air is typical. Lately oxygen or oxygen and air roasting, at low temperatures, have showed considerable promise. Other commercial ore treatment methods prior to cyanidation are high pressure oxygen and/or oxygen-ozone pretreatment, chlorine pretreatments, hypochlorite pretreatments and the like.
To improve cyanidation of ores during such cyanidation ozone, or ozone and oxygen, or oxygen, or a surfactant, or combinations of these are also employed. In the instance of gold recovery, methods such as "carbon-in-pulp" (or "CIP") and "carbon-in-leach" (or "CIL") are used to improve cyanidation reactions and gold recovery.
However, cyanidation has certain shortcomings, primarily an ore material must be neutralized after an acid generating treatment as cyanidation must be carried out on the alkaline side of the pH scale; likewise high cyanide consumption renders a process less attractive. When using thiourea, neutralization of the ore is not as demanding and does not affect thiourea extraction of gold, but the extraction economies are impaired by the higher cost of thiourea and the reduced efficiency when compared with cyanide.
Other compounds which have been used and offer promise because of reagent costs are compounds such as thiosulfates of which ammonium thiosulfate is one of the desirable candidates. Although still other materials are used for gold recovery, these are not yet of industrial significance.
When ammonium thiosulfate and the like are used, neutralization of ore is required as appropriate pH ranges are neutral to alkaline, e.g. to about pH 7 to 10 and preferably to at least about 9. As pyritic sulfidic ores and many other ores need to be neutralized because of the acidity of these ores when subjected to oxygenation or biooxidation and like treatments, separate process steps are required.
Inasmuch as gold is occluded in the sulfide matrix of the ore, the accessibility by cyanide has sought to be improved for these ores; the same is also true when considering an appropriate sulfide, e.g., pyrite for oxidation or biooxidation. Although various oxidation or biooxidation reactions have been tried such as vat, autoclave, slurry or liquid solution oxidations, these reactions are not practical when using large ore bodies having low gold content. As one of the approaches to oxidation of low content metal sulfide ores, biooxidation has come into prominence and much effort has been expended in research. Biooxidation was first applied to copper. Biooxidation of copper ore has been a well tried method although it is considered fairly slow.
When biooxidation is coupled with oxidative bioleaching, i.e. when direct, indirect and even galvanic leaching reactions are involved, some of the disadvantages of the slow biooxidation reactions are mitigated. Biooxidation reactions typically involve arsenopyritic and pyritic iron sulfide-containing ores including those that have some refractory carbon components present. Biooxidation, however, can suffer from inhibitory concentrations of some metals present in the ore. Biocidically active metals are such as arsenic, antimony, cadmium, lead, mercury, molybdenum. Ions such as chlorine, bromine and the like affect the biooxidation processes. Because of slow growth rates for some bacteria as well as temperature variations in a typical ore dump undergoing sulfide oxidation, considerable efforts have been expended to improve the rate constraints which have limited or held back the potentially very useful application of biooxidation.
Hence, considerable investigation has been made of the various limiting conditions concerning commercial biooxidation including such factors as ores in heaps or in slurry form, the use of surfactants, the use of potentiators or biooxidation promoters such as silver, aluminum, etc., appropriate selection and growing of robust bacteria which would be resistent to the inhibitory biocide activity of metals such as arsenic and growing the bacteria in profuse amounts. Other considerations have been such as nutrient access, air access and carbon dioxide access for making the process even more efficient and thus an attractive ore treatment option. References illustrating these efforts are such as by Bartlett, Aeration Pretreatment of Low Grade Refractory Gold Ores, Minerals and Metallurgical Processing, pp 22-29, (Feb. 1990); Bennett et al, Limitations on Pyrite Oxidation Rates in Dumps Set By Air Transport Mechanisms, Biohydrometallurgy, Proceedings of Jackson Hole Symposium, Aug. 13-18, 1989 Canmet (1989); Burbank et al, Biooxidation of Refractory Gold Ore in Heaps, Ch. 16, pp 151-159 in Advances in Gold and Silver Processing, Reno Proceedings of Symposium "Goldtech 4", Reno, Nev., Sep. 10-12, 1990, Society of Mining, Metallurgy and Exploration, Publisher, 1990; Dix, Laboratory Heap Leach Testing: How Small and Large Scale Tests Compare, Mining Engineering, Jun. 1989, Pages 440-442.
Amongst the methods seeking to improve biooxidation many methods have been proposed for mechanically increasing the access of the biooxidant bacteria to the ore. These methods have relied upon agitation of the ore either in tanks, slurries, providing circulation in vessels or reconstitution and remixing of the materials including stirring, raking, forming an improved slurry, transfer of slurry materials, providing stirred tank basins or have addressed various aspects of heap construction and utilization. References to such considerations are found in an article by Andrews, Large-Scale Bioprocessing of Solids, Biotechnology Progress, Vol. 6, pp 225-230, 1990.
Patents which illustrate some of these methods mentioned above are found such as in U.S. Pat. No. 4,324,764 concerning mechanical distribution of ores or distribution of ores by conveyors such as in U.S. Pat. No. 4,571,387 or a change in heap structure such as in U.S. Pat. No. 4,279,868 or stagewise heap formation such as in U.S. Pat. No. 4,017,309; or a stirred tank--semi "heap" construction such as disclosed in U.S. Pat. No. 4,968,008.
However, when treating large amounts of waste heap material or tailing material, the normal considerations that are applicable in high grade precious metal ore treatments are not viable. For waste ore treatment, economics often dictate a one-shot type of heap formation, e.g. for the depth, the size, the reactant accessibility, etc. Moreover, for biooxidation, the induction times concerning biooxidants, the growth cycles, the biocide activities, viability of bacteria and the like become important because the variables such as accessibility, particle size, settling, compaction and the like are economically irreversible once a heap has been constructed as such heaps cannot be repaired except on a very limited basis. For example, compaction problems such as are encountered in heap treatment of ores, and others such as puddling, channelling, or nutrient-, carbon dioxide-, or oxygen-starving, uneven biooxidant bacterial distribution, and the like have been addressed in a number of investigations with respect to biooxidation. Such problems are also encountered in cyanide leaching.
For example, to solve channelling in percolation leaching by cyanides it is known to agglomerate the ore materials of high grade ores such as disclosed in U.S. Pat. Nos. 4,256,705 and 4,256,706. Other approaches to improve percolation leaching by cyanides include addition of fines such as flocculating materials, fibers, wood, pulp and the like as disclosed in U.S. Pat. No. 4,557,905. The last patent discloses leachable matrix formation to allow for access of cyanide to the precious metal values.
An ultimate, albeit impractical, suggestion for cyanide leaching has been found in U.S. Pat. No. 4,424,194 which shows making useful articles and then leaching these. This patent may have as its progenitor the early U.S. Pat. No. 588,476 of Aug. 17, 1887, which discloses porous casts made of gold "slimes" and gypsum. These casts are thereafter broken and leached.
Although for a variety of different reasons agglomeration has been practiced in the metallurgical arts such as in high temperature blast furnace art for various feed material preparations for blast furnaces, opposite suggestions have also been found concerning non-agglomeration and extraction of metals such as the pulp-liquid extraction described in U.S. Pat. No. 3,949,051. Extraction of the precious metals from heaps, preformation of heaps and heap treatment is found such as in U.S. Pat. Nos. 4,017,309 and 4,056,261.
Further improvements for access of cyanide to the precious metals have been described in U.S. Pat. Nos. 4,318,892 and 4,279,868 as well as U.S. Pat. No. 4,301,121. All of these attempts have sought to improve the distribution of the leachant or the mixing ratios of the ore to the lixiviant, but these attempts are typically addressed to providing better access for cyanide and to overcome the ostensible refractoriness of the ore. Other like disclosures have been found in U.S. Pat. Nos. 4,324,764 and 4,343,773.
Heap improvements have been found in the construction of the particles such as paste formation with the lixiviant and subsequent ageing of the ore on treatment of the same, described in U.S. Pat. No. 4,374,097. Likewise, specific berm construction for the improved extraction of liquids from a specifically constructed heap has been found in U.S. Pat. No. 4,526,615. Similarly various particle specifications have been described for the ore particle treatment including the micro agglomerates of a size of 500 microns (and lower) found in U.S. Pat. No. 4,585,548.
In all of these heap formations, heap treatments or heap leaching methods, shortcomings have been sought to be overcome by the increase of cyanide efficiency such as by oxygen addition, e.g. in U.S. Pat. No. 4,721,526, or the use of various liquors in the recovery of gold described in U.S. Pat. No. 4,822,413.
Agglomerating agents for copper ores are shown in U.S. Pat. No. 4,875,935. Opening up clogged heaps has also been shown and discussed in U.S. Pat. No. 3,819,797 and heap treatment for distribution of a lixiviant is disclosed in U.S. Pat. No. 5,005,806. Finally, both conjoint crushing and agglomeration of ore has been discussed in U.S. Pat. No. 4,960,461.
Attempts have been made to ameliorate the compaction and imperviousness that results when ore materials containing clays and/or fines are heaped. Clays and fines pose difficulties in hydrometallurgical processes used in the recovery of metal values from ore materials. In order that an inappropriate accumulation of clays and fines does not hinder the flow of process liquor through a heap of ore material during heap leaching, clays and fines need to be immobilized and uniformly distributed in the heap, such as by agglomeration with larger particles of ore material. Unfortunately, the percolation of process liquor through the heap has a tendency to free clays and fines from the agglomerate and to result in the segregation of clays and fines from agglomerates and their migration into a nonuniform distribution in the heap. This loose clay and fine material can concentrate in pore spaces and plug flow channels in the heap. In addition, the pH of the environment in the heap can exacerbate this problem since pH has an effect on the swelling of clays and the stability and solubility of components of the ore material. In this regard, see Kurtz, J. P., et al. Clay Problems Encountered in Gold Heap Leaching (manuscript submitted to Society of Mining Engineers for the Symposium on "Small Mines Development in Precious Metals" Aug. 30-Sep. 2, 1987) and Chamberlin, P. D., "Agglomeration: Cheap Insurance for Good Recovery When Heap Leaching Gold and Silver Ores", Mining Engineering 1105-1109 (December 1986.)
One attempted solution to this problem is the addition of cement as a binding agent and lime as a pH control in caustic cyanide leaching environments resulting in a high heap pH. While a high pH itself poses problems with respect to swelling and undesired solubilization of minerals, biooxidation requires an acid pH--an environment at odds with the pH produced by the cement/lime approach to agglomeration.
Anionic copolymers of acrylamide and acrylic acid have found use as described in U.S. Pat. Nos. 5,077,021 and 5,077,022 as agglomerating agents in heap leaching with caustic cyanide leachants--a strongly alkaline environment that is inhospitable to biooxidative microbes, such as Thiobacillus ferroxidans. Reconstituting an ore heap in order to agglomerate the ore material with an alkali-tolerant agglomeration aid before caustic cyanide leaching is not as economically efficient as being able to prepare a heap, decrease its sulfide refractoriness and then leach desired metal values--without needing to reconstitute the heap to change agglomeration agents.
U.S. Pat. No. 3,418,237 issued Dec. 24, 1968, describes the use of water-soluble acrylic polymers in settling ore pulps that contain only up to 25% clay. The described polymers are stated to be completely ineffective in settling mineral suspensions when the amount of clay materials present exceeds about 25%.
U.S. Pat. No. 4,875,935, issued Oct. 24, 1989, discusses the use of anionic poly(acrylamide) polymers, including poly(acrylamide) copolymers with acrylic acid, methacrylic acid and itaconic acid, in the agglomeration of copper ores for heap leaching with dilute sulfuric acid. However, this approach ignores the issues raised by microbial biooxidation and microbial viability.
U.S. Pat. No. 4,898,611, issued Feb. 6, 1990, discusses the use of water-soluble vinyl addition polymers, such as poly(acrylamide) and its water-soluble acrylic acid, methacrylic acid, itaconic acid, acrylonitrile and styrene copolymers including cationic and anionic polymers, as agglomeration agents for heap leaching with a cyanide lixiviant. In this regard this approach is subject to the same deficiencies as U.S. Pat. Nos. 5,077,021 and 5,077,022 discussed earlier.